tests: disable CRL checks from gpgsm

tests: disable CRL checks from gpgsm

GPGME has a strange failure mode when it is in offline mode, and/or
when certificates don't have any CRLs: in particular, it refuses to
accept the validity of any certificate other than a "root" cert.

This can be worked around by setting the `disable-crl-checks`
configuration variable for gpgsm.

I've reported this to the GPGME upstream at
https://dev.gnupg.org/T4883, but I have no idea how it will be
resolved.  In the meantime, we'll just work around it.

Note that this fixes the test for verification of
id:smime-multipart-signed@protected-headers.example, because
multipart/signed messages are already handled correctly (one-part
PKCS#7 messages will get fixed later).

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>